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Elite on the BBC Micro

Keyboard: TT17X [Master version]

Name: TT17X [Show more] Type: Subroutine Category: Keyboard Summary: Scan the digital joystick for movement
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * DJOY calls TT17X

Scan the digital joystick for stick movement, and return the result as deltas (changes) in x- and y-coordinates as follows: * For joystick, X and Y are integers between -2 and +2 depending on how far the stick has moved Returns: X Change in the x-coordinate according to the joystick movement, as an integer (see above) Y Change in the y-coordinate according to the joystick movement, as an integer (see above)
IF _COMPACT .TT17X LDA VIA+&60 \ Read the 6522 User VIA, which is where the Master \ Compact's digital joystick is mapped to. The pins go \ low when the joystick connection is made, so we need \ to check whether any of the following are zero: \ \ PB0 = fire \ PB1 = left \ PB2 = down \ PB3 = up \ PB4 = right LSR A \ Shift the PB0 bit out, as we aren't interested in the \ fire button in this routine LDX #0 \ Set the initial values for the results, X = Y = 0, LDY #0 \ which we now increase or decrease appropriately LSR A \ If PB1 from the User VIA is set (left), skip the BCS P%+3 \ following DEX \ Set X = X - 1 LSR A \ If PB2 from the User VIA is set (down), skip the BCS P%+3 \ following INY \ Set Y = Y + 1 \ \ Note that this is the opposite direction to the stick \ direction, as moving the stick down should decrease Y. \ To fix this, we flip this result below LSR A \ If PB3 from the User VIA is set (up), skip the BCS P%+3 \ following DEY \ Set Y = Y - 1 \ \ Note that this is the opposite direction to the stick \ direction, as moving the stick up should increase Y. \ To fix this, we flip this result below LSR A \ If PB4 from the User VIA is set (right), skip the BCS P%+3 \ following INX \ Set X = X + 1 LDA JSTE \ JSTE contains &FF if both joystick channels are BEQ TT171 \ reversed and 0 otherwise, so skip to L7F80 if the \ joystick channels are not reversed TXA \ The X channel needs to be reversed, so negate the EOR #&FF \ value in X, using two's complement CLC ADC #1 TAX .TT171 LDA JSTE \ Set A to &FF if both joystick channels are reversed \ and 0 otherwise EOR JSTGY \ JSTGY will be 0 if the game is configured to reverse \ the joystick Y channel, &FF otherwise, so A will be \ 0 if one of these is true: \ \ * Both channels are normal and Y is reversed \ * Both channels are reversed and Y is not \ \ i.e. it will be 0 if the Y channel is configured to be \ reversed BEQ TT17X-1 \ If the result in A is 0, return from the subroutine \ (as TT17X-1 contain an RTS), as we already set the Y \ value above to the opposite direction to the stick \ If we get here, then the configuration settings are \ not set to reverse the Y channel, so we now negate the \ Y value, as we set Y above to the opposite direction \ to the stick TYA \ The Y channel should be reversed, so negate the value EOR #&FF \ in Y, using two's complement CLC ADC #1 TAY \ Fall through into ECMOF to return from the subroutine ENDIF